This invention relates to high frequency control of a power field effect transistor and in particular to an improved resonant gate drive circuit for a power MOSFET.
It is known in the prior art of power converters that a gate drive circuit coupled to a power MOSFET contributes to an efficiency reduction of the power converter especially at high frequencies so various driver schemes have been developed. The gate drive power loss is proportional to the switching frequency and is a major limitation in the design of high-efficiency power converters in the MH.sub.Z region. One approach to improve the gate drive power loss is to use a resonant gate drive circuit as described in a paper entitled "A MOS Gate Drive With Resonant Transitions," by Dragan Maksimovic, 22nd Annual IEEE Power Electronics Specialists Conference (PESC), Jun. 23-27, 1991, pp. 527-532. Maksimovic describes a gate drive that provides quasi-square-wave gate-to-source voltage with low impedance between gate and source terminals in both On and Off states. The equivalent gate capacitance of the power MOS transistor is charged and discharged in a resonant circuit so that energy stored in the equivalent gate capacitance is returned back to the source. However, two other sources of gate drive power losses still remain and they are cross conduction and linear operation. Cross conduction occurs during transitions between the On and OFF states of the gate drive transistors. Their simultaneous conduction creates a momentary short circuit of the gate drive power supply. Linear operation losses in the gate drive transistors occur during turn-on and turn-off transitions due to an overlap of drain-to-source voltage and drain current. The larger the equivalent gate capacitance, the longer the gate drive transistors will stay in the linear region.
Another approach for improving gate drive power loss is described in NASA Technical Brief entitled "Recovering Energy from a Rapidly Switched Gate," by Wally E. Rippel, Vol. 14, No. 9, May 1990. A circuit for recovering the energy normally lost by charging and discharging equivalent gate capacitance of field effect transistors, and the like, during high frequency switching is described comprising a special pulse forming logic circuit, a complementary metaloxide semiconductor (CMOS) driver, and a resonating inductor in series with the gate, wherein the logic circuit furnishes a zero-voltage notch of specific length and placement near the beginning and end of the usual square drive signal, whereby the capacitance is resonantly charged and discharged, its energy recovered, and further whereby higher frequency switching is enabled. However, the elaborate pulse-forming logic circuit produces a tolerance-sensitive control signal resulting in production adjustments. Also, cross conduction losses are not addressed by this circuit.
A circuit for high speed control of field effect power transistors comprising a transformer with a primary winding and a secondary winding with an energy storage structure coupled to the secondary for storing energy required for controlling the gates of the field effect power transistors is described in U.S. Pat. No. 4,767,952, issued to Michael Nollet on Aug. 30, 1988. This invention describes a fast transformer-isolated gate drive circuit. Nollet overcomes speed limitation inherent in conventional transformerisolated circuits by inserting an energy storing and signal receiving element between the transformer and the gate of the power MOSFET. While this circuit allows increasing switching speeds without increasing the gate drive current, it does not improve the capacitive power dissipation and cross conduction and linear operation losses in the gate drive transistors.